On July 12, 1998, at 0800 central daylight time, a Grumman G-164B, N6731K, piloted by a commercial pilot, received substantial damage on impact with terrain during a forced landing following a total loss of engine power, near New Haven, Illinois. The flight was operated under 14 CFR Part 137. Visual meteorological conditions prevailed at the time of the accident. The pilot sustained no injuries. The flight departed from a private airport, near Mount Vernon, Indiana, at 0700, on an aerial application flight and was returning to Mount Vernon, Indiana. The pilot reported that he had arrived at his job site at 0700 to spray 70 acres of corn. He had completed spraying and was returning to his airstrip at an altitude of 150 ft above ground level for another load when the engine torque reading dropped to 30%. While attempting to increase power, the pilot reported that the engine quit at which time he then preformed an emergency landing into a muddy field. The pilot stated that there was 90 gallons of fuel on board upon departure and that he did nor hear any unusual engine sounds during the loss of engine power. Inspection of the engine by a Federal Aviation Administration Inspector found the compressor, turbine, accessory gear box and reduction gear box to rotate. The chip detector was examined and was found to be free of contaminants. The fuel lines to the engine, the fuel filter and its respective fuel bowl was unobstructed. No external contaminants were observed to be present on the igniter. The engine pneumatic lines were in place and did not possess any indications of leakage. One blade of the one the first stage turbine rotor and two blades of the power turbine were fractured. The compressor and turbine housing exhibited twisting without any localized outward deformations or punctures. The fuel-oil heater was found to be intact and upon internal examination a 50/50 ratio of fuel to water was estimated to be present. The high pressure fuel pump and fuel control units were examined by Pratt and Whitney under the supervision of the Transportation Safety Board of Canada and no anomalies were noted.